1. Field of Invention
The present invention relates to a device and method for a vascular valve for the vascular system and more specifically for the venous vascular system of the human body and a method for forming the venous valve and more specifically for forming the venous valve from a native vein that may have been enlarged diametrically within the patient causing the naturally occurring venous valve to become incompetant.
2. Description of Prior Art
Venous valves are found within native venous vessels and are used to assist in returning blood back to the heart in an antegrade direction from all parts of the body. The venous system of the leg for example includes the deep venous system and the superficial venous system, both of which are provided with venous valves which are intended to direct blood toward the heart and prevent backflow or retrograde flow which can lead to blood pooling or stasis in the leg. Incompetent valves can also lead to reflux of blood from the deep venous system to the superficial venous system and the formation of vericose veins. Superficial veins which include the greater and lesser saphenous veins have perforating branches in the femoral and popliteal regions of the leg that direct blood flow toward the deep venous system and generally have a venous valve located near the junction with the deep system. Deep veins of the leg include the anterior and posterior tibial veins, popliteal veins, and femoral veins. Deep veins are surrounded in part by musculature tissue that assist in generating flow due to muscle contraction during normal walking or exercising. Veins in the lower leg have a static pressure while standing of approximately 80-90 mm Hg and this pressure can be reduced during exercise to 60-70 mm Hg. Despite exposure to such pressures, the valves of the leg are very flexible and can close with a pressure drop of less than one mm Hg. Due to the endothelial covering on the venous valves, they are able to remain patent and resist thrombosis with blood flow rates of less than 50 ml/min found typically in some of the smaller veins of the lower leg. Although the present invention has direct application to the treatment of venous valvular dysfunction of the leg, it is understood that the invention is not limited to this application and can be applied equally well to the treatment of veins throughout the human body as well as other tubular elements of the body requiring a valve.
Veins typically in the leg can become distended from prolonged exposure to excessive pressure and due to weaknesses found in the vessel wall causing the natural venous valves to become incompetent leading to retrograde blood flow in the veins. Such veins no longer finction to help pump or direct the blood back to the heart during normal walking or use of the leg muscles. As a result, blood tends to pool in the lower leg and can lead to leg swelling and the formation of deep venous thrombosis and phlebitis. The formation of thrombus in the veins can further impair venous valvular function by causing valvular adherence to the venous wall with possible irreversible loss of venous function. Continued exposure of the venous system to blood pooling and swelling of the surrounding tissue can lead to post phlebitic syndrome with a propensity for open sores, infection, leading to possible limb amputation.
Repair and replacement of venous valves presents a formidable problem due to the low blood flow rate found in native veins, the very thin wall structure of the venous wall and the venous valve, and the ease and frequency of which venous blood flow can be impeded or totally blocked for a period of time. Surgical reconstruction techniques used to address venous valve incompetence involve venous valve bypass using a segment of vein with a competent valve, venous transposition to bypass venous blood flow through a neighboring competent valve, and valvuloplasty to repair the valve cusps. These surgical approaches are described in medical journals and in standard surgical text books. These surgical techniques are highly technique dependent and difficult to perform by a highly trained vascular surgeon.
The presence of a low or intermittent blood flow rates found in the veins of the lower leg requires that any suitable venous valve replacement contain an endothelial covering to protect the vessel against thrombosis. Blood stoppage for a period of time in contact with most foreign material can result in thrombus formation, and ensuing failure of any venous valve constructed of a polymeric material or some biomaterials.
Quijano describes in U.S. Pat. No. 5,500,014 the use of a biological valvular prosthesis that is obtained from the jugular vein of an animal. The valve is chemically fixed to give it strength. The fixing process tends to cause such valves to be stiff and calcification has been known to occur at flexation sites. Another problem with valves of this type is their lack of forming a stable endothelium with the resulting formation of thrombus when blood flow is impeded or temporarily blocked. Others have tried constructing venous valves out of a biological tissue material obtained from another species or the same species. Some tissues that have been used include pericardium and venous tissue treated with a crosslinking treatment. These devices have suffered problems associated with calcification, tissue degradation, tissue rejection, acute thrombosis, long term thrombosis, and valvular failure due to mechanical dysfunction.
Another vascular prosthesis that is constructed out of polymeric or metallic components is described by Camilli is U.S. Pat. No. 5,358,518. He describes a movable rigid or semi-rigid plate that pivots and allows unidirectional blood flow through the venous tube. The biomaterials used in the device of Camilli will not form a stable endothelium on the blood flow surface; due to the low blood flow and often times interrupted blood flow found within the venous system, this device will be prone to thrombosis and failure.
Laufer (U.S. Pat. No. 5,810,847) describes an appliance that is constructed out of a biomaterial that is attached to the cusps of an existing venous valve. Such a polymeric appliance will have a propensity to thrombosis in the low blood flow conditions found in the venous system. Also, many patients with venous problems do not have suitable valves onto which the appliance described by Laufer can be attached.
Lane describes in U.S. Pat. Nos. 5,147,389 and 4,904,254 and Shifrin describes in U.S. Pat. No. 5,476,471 devices that are intended to surround the outer surface of incompetent or insufficient venous valves. These devices are intended to reduce the diameter of the vessel in the region of the incompetent valve allowing the natural cusps to approximate each other leaving the valve commissures intact. For these devices to work properly, the valves of the vein must not be adherent to the vessel wall, this adherent condition is often found when the vessel is exposed to deep venous thrombosis. This significantly limits the patient population that can benefit from a device of this type.
A treatment for venous valvular dysfunction in patients that have had their vein wall distended and their valves irreversibly damaged is needed. The treatment should involve autologous tissue such that an endothelial layer is present in blood contact and thrombosis due to low flow is not of concern. The treatment should be easily performed so that patients with tissue edema and ulcers can tolerate the intervention and heal the area being accessed. The treatment should be applicable to those patients who have had deep venous thrombosis, phlebitis, or other vascular trauma and have irreversibly lost venous valvular function.